KEY WEST, Fla. — Tropical Storm Isaac lashed south Florida with winds and heavy rain on Sunday after battering the Caribbean, threatening to interrupt most U.S. offshore oil production in the Gulf of Mexico and disrupting plans for the Republican National Convention in Tampa.

A hurricane warning was issued for the Gulf Coast from Louisiana to the Florida Panhandle, including New Orleans, which was devastated when Katrina struck the city on Aug. 29, 2005, killing more than 1,800 people and causing billions of dollars of damage to the Gulf Coast.

In Louisiana, Gov. Bobby Jindal declared a state of emergency and said 15 low-lying parishes outside New Orleans' newly built, $14.5 billion flood defense system would likely be under mandatory evacuation orders by Monday.

“There's really nothing that's going to stop this storm from forming and from strengthening,” said Jindal, a seasoned crisis manager who has weathered such disasters as the 2010 BP oil spill.

On Mississippi's Gulf Coast, residents started stocking up on supplies and securing their homes. “It is packed.

They are clearing the shelves,” said Lindy Stewart after shopping at a Sam's Club in Gulfport. Stewart said she bought bread, lunch meat and other “stuff you need to survive a couple of days without power.”

The Mississippi State Port Authority ordered the port of Gulfport cleared of all cargo vessels and cargo containers.

Isaac is forecast to become a hurricane either late Monday or Tuesday.

The NHC said Isaac was expected to eventually intensify to a Category 2 hurricane with “extremely dangerous” sustained winds of 105 miles per hour as it swept up the warm waters of the Gulf of Mexico.

Forecasters are predicting a more westward track that could bring Isaac over the heart of the U.S. offshore oil patch, which produces about 23 percent of U.S. oil output and 7 percent of its natural gas output.

Meteorologists at Weather Insight, an arm of Thomson Reuters, predict the storm will spur short-term shutdowns of 85 percent of the U.S. offshore oil production capacity and 68 percent of the natural gas output.

With the threat to offshore oil infrastructure and Louisiana refineries, U.S. crude oil prices traded up 75 cents to $96.90 a barrel in Asia trading early Monday.

Once ashore, the storm could wreak havoc on low-lying fuel refineries along the Gulf Coast that account for about 40 percent of U.S. refining capacity.

That could send gasoline prices spiking just ahead of the Labor Day holiday, analysts said.

“It's going right in the heart of refinery row,” Phil Flynn, an analyst with Price Futures Group in Chicago, said on Sunday.